1. Field of the Invention
This invention relates to automatic focus adjusting devices, and more particularly to active type automatic focus adjusting devices for automatically focusing an optical system by projecting a signal onto an object and detecting the reflection of the signal therefrom.
2. Description of the Prior Art
The present inventor and others have previously proposed an automatic focus adjusting device for video cameras. The video camera is equipped with an active type automatic focusing mechanism using a light source in the form of a near infrared light emitting diode. This invention is disclosed in U.S. patent application Ser. No. 536,369 (corresponding to Japanese patent application No. SHO 57-171923). The device utilizes the fact that the high frequency component of a video signal is capable of reaching a peak when the object image is in focus. After the focus adjusting operation of the active type automatic focusing mechanism has been completed, the variation of the high frequency component of the video signals is monitored to determine whether the image for the subsequent frame is in focus or out of focus. If the image has gone out of focus, the active type automatic focusing mechanism is again rendered operative to reapply the focus adjusting operation. The use of this device provides the possibility of re-establishing an in-focus condition even when the object has moved after an initial focus adjustment, as soon as the image becomes out of focus. Additionally, it makes it unnecessary to continue energization of the light emitting diode after the image is in focus. This reduces wasteful consumption of electrical power.
However, in this device, after the image for the first field has been focused, determination of whether the image for the second field is in focus or out of focus is made based only on the value of the high frequency component of the video signal. This results in a drawback. The latter occurs, for example, as the camera is panning, and detects another object image. The detection during panning increases the high frequency component of the video signal. Accordingly, despite the fact that the actual image is out of focus, the system perceives it to be in focus. This rendering it impossible to perform an accurate automatic focus adjustment.